Procedures for Repair Welding and Surfacing

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Procedures for Repair Welding and Surfacing

Repair welding involves three key operations: preparation for welding, repair welding, and post-weld operations. This article introduces the necessary actions and procedures for effective repair welding and surfacing.

1. Preparation for Welding

Before starting the welding process, several factors must be considered:

Safety
The welding area must be surveyed to address all safety considerations. This includes posting safety signs, removing combustible materials, and draining fuel tanks from equipment such as aircraft, boats, and trucks. Additionally, toxic materials like lead paint and plastic coverings should be eliminated.

Cleaning
The work area must be free from contaminants, including dirt, grease, oil, rust, and paint. The cleaning method depends on the material to be removed and the workpiece location. Recommended methods include steam cleaning, solvent cleaning, abrasive blast cleaning, and power tool cleaning. Investing time in cleaning will yield long-term benefits.

Disassembly
For most repair jobs, disassembly may be necessary. This may involve removing lubrication lines, instrument tubing, and wiring, and, in some cases, major components from machinery frames.

Protection of Adjacent Machinery
Parts that are not removed should be protected from weld spatter and sparks using sheet metal guards or baffles. Asbestos cloth can safeguard machined surfaces, and protective materials should be secured with wire, clamps, or temporary bracing. Protect all machined surfaces within five feet of the welding operation.

Bracing and Clamping
Complex repairs may require bracing or clamping due to the weight of parts or external loads. Temporary braces may be welded to the structure being repaired.

Layout for Repair Work
A layout is necessary to indicate the metal to be removed for a full-penetration weld. Minimize the amount of metal removed while ensuring that welding can be performed comfortably.

Preheating
Preheating is essential when flame cutting or gouging, similar to welding. It helps prevent thermal shock on the metal.

Cutting and Gouging
Oxygen-fuel gas cutting torches are commonly used for cutting and gouging. Special gouging tips should be selected based on joint preparation geometry. Flame gouging can help identify and follow cracks during the process. For some metals, mechanical chipping or grinding may be more suitable.

Grinding and Cleaning
The resulting surfaces may require grinding to achieve clean, bright metal. For critical work, magnetic particle inspection is advisable to ensure all cracks and defects are removed.
These steps are crucial for proper weld preparation; while some may be omitted, all should be considered.

2. Repair Welding

Successful repair welding requires a logical sequence of operations:

Welding Procedure
A detailed welding procedure must be available, outlining the welding process, filler metals, preheat requirements, and joint technique.

Welding Equipment
Ensure sufficient welding equipment is on hand to avoid delays, including electrode holders, grinders, wire feeders, and cables.

Materials
Adequate materials must be available, including properly stored filler metals, insert pieces, and reinforcing components. This also includes fuel for preheating and shielding gases if required.

Alignment Markers
Prior to welding, alignment markers (e.g., center punch marks) should be made across the joint.

Welding Sequences
The welding procedure should detail the sequence to be followed, which may include block welding, back-step welding, or peeling.

Safety
Maintain safety measures throughout the welding operation, including adequate ventilation when using fuel gases for preheating.

Weld Quality
Continuously check weld quality. The final weld should be smooth, with no notches, and any reinforcing should blend seamlessly into the existing structure.

3. Post-Weld Operations

After completing the weld, allow it to cool slowly, avoiding exposure to wind or drafts. Machinery loads should not be applied until the temperature returns to normal.

Inspection
Inspect the finished weld for quality and smoothness, employing nondestructive testing methods such as magnetic particle, ultrasonic, or X-ray testing.

Clean-Up Operation
Remove all strong backs, bracing, and protective covers. Ensure all weld stubs, spatter, slag, and dust are cleaned thoroughly to prevent future issues.

Repainting
After cleaning, repaint the welded area and re-grease other components in preparation for machinery operation.

Reassembly
Return and reassemble any machinery components that were disassembled during the repair process.

Rebuilding and Overlaying

Rebuilding and overlaying with weld or spray metal are considered surfacing operations. Surfacing deposits filler metal on a base metal to achieve desired dimensions or properties, while overlaying involves applying a deposit with specific properties that may differ from the original surface.

Rebuilding restores parts to their original dimensions and properties, while overlay surfacing enhances wear resistance and corrosion protection.

Surfacing can be achieved using various welding processes or thermal spraying, with the process selection based on specific project requirements. Thermal spray methods are advantageous when minimal heat input is essential, especially for materials that cannot be deposited through traditional welding.

Selection of the Welding Process

Choosing the appropriate welding process and technique is as crucial as selecting the deposit alloy. Common methods for hardfacing include:

• Shielded Metal Arc Welding (SMAW): Widely used for hardfacing, applicable in various positions and suitable for both small and large parts.
• Submerged Arc Welding (SAW): Primarily used in plant operations for flat-position welding and routine applications.
• Flux-Cored Arc Welding (FCAW): A popular semiautomatic process that can be used in the field or shop and is not limited to the flat position.
• Gas Tungsten Arc Welding (GTAW): Suitable for smaller applications and typically used in the shop for precision work.
• Plasma Arc Welding (PAW): Effective for certain applications requiring higher temperatures, often limited to smaller jobs.
• Electroslag Welding: Used for specialized applications, including rebuilding components like crusher hammers.
• Oxyacetylene Welding: Applied for specialized alloys on thin edges.

The selection process should consider normal factors alongside specific comments mentioned above. Once the process is chosen, select the deposited metal to ensure it provides the necessary properties for the application.